100 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



and the thoracic part of the nervous system ; wliile, in the 

 abdomen, its strands are attached to the sternal exoske- 

 leton of all the somites and extend, on each side of the 

 rectum, to the telson. 



When the exoskeleton is cleaned by maceration, the 



Fig. 22. — Astaovs fluviatil'is. — A longitudinal section of the body to 

 siiow the principal muscles and their relations to the exoskeleton 

 (nat. size), a, the vent ; add.m, adductor muscle of mandible ; 

 e.m, extensor, and/.m, flexor muscle of abdomen ; oe.i, oesophagus ; 

 pep, prooephalic process ; i,t', the two segments of the telson ; 

 XV — XX, the abdominal somites ; 1 — SO, the appendages ; n , >•. , 

 hinges between the successive abdominal somites. 



abdomen has a slight curve, dependent upon the form and 

 the degree of elasticity possessed by its different parts ; 

 and, in a living crayfish at rest, it will be observed that 

 the curvature of the abdomen is still more marked. 

 Hence it is ready either for extension or for flexion. 



A sudden contraction of the flexor muscles instantly 

 increases the ventral curvature of the abdomen, and 



